Various procedures are known for mutual arrangement and alignment of the contact surfaces of two substrates, for example wafers, especially nontransparent wafers.
One known procedure is the use of two pairs of microscopes which are each calibrated to a certain viewing point. For purposes of alignment, first the lower wafer is moved to under the upper microscopes and the microscopes are aligned to the lower wafer, the position is fixed and the two alignment keys of the wafers are stored. Then the upper wafer is aligned to the stored alignment keys using the lower microscopes. Then the lower wafer is moved into its original position and the wafers are contacted. With the above described method high precision can be achieved in the positioning. The system however works only based on the detected relative positions of the two alignment keys on both wafers to one another so that calibration of the microscopes to one another and the movement of the wafers in alignment can lead to errors in alignment. Furthermore the number of measurement points on the wafer is limited. The above described method is described in U.S. Pat. No. 6,214,692.
Another approach is to arrange two pairs of microscopes between the wafers to be contacted in order to align the two alignment keys opposite one another, then to move the microscopes out and to finally move the wafers exactly onto one another. In this connection the corresponding errors can occur by the relative motion of the wafers to one another and the relative detection of the alignment keys.
The alignment accuracy of the known alignment technologies is in the region of 0.5 μm, the distribution of the structures which are located on the wafers and which can be aligned to one another, for example chips, and possible deviations of the chips from the given or nominal positions on the wafer not being considered so far. The growing interest in 3D integration reduces the spacing and size of the bore holes so that there is a great demand for more accurate alignment. The deviation from the nominal position of the alignment structures has been ignored to date since the adjustment accuracy which has been possible to date was far more than 10 times these deviations. The deviations are generally less than 100 nm.
One major problem of the existing approaches is the mechanical accuracy of the movements of the components toward one another.
Another problem consists in the optical detection accuracy based on the required working distance of the optics from the wafers. In typical alignment devices (for example U.S. Pat. No. 6,214,692) the working distance must be large enough to be able to move the holding devices for the substrates between the optics. The necessity of this distance limits the maximum usable magnification of these microscopes and thus the maximum attainable detection accuracy for the alignment keys and subsequently the alignment accuracy.
In an arrangement of the optics between the wafers, the orthogonal alignment of the optics to the contact surfaces of the wafer is another aspect which leads to faults in the micron or nanometer range.